Structural Constraints, Spandrels, and Exaptation 1259
of this chapter, but I raise three points here to set a framework of plausibility for
frequencies far too high to ignore.
RELATIONSHIPS TO GEOMETRY AND ARCHITECTURE. As Geoffrey and
other formalist thinkers recognized from the inception of evolutionary studies in
biology, organisms are integrated entities, not hodge-podges of independent attributes
each dedicated to a separate function. For two major reasons, this evident and
venerable notion implies a great importance and high relative frequency for
spandrels. First, any change in one part of the body must propagate correlated
alterations to other parts. Selection may generate the original change for adaptive
reasons, but many automatic consequences will probably be spandrels. Second, any
adaptive feature of one organ will also express inherent and ineluctable attributes that
must rank as spandrels. Most of these sequelae, although surely more numerous than
adaptive aspects of the same feature, will probably remain forever irrelevant to
evolutionary success of the lineage. (Bones are made of calcite and apatite for
adaptive reasons, but bones are also white because the chemistry of these compounds
so dictates. Invisible during life, this spandrel property of whiteness will probably
never influence the evolutionary history of the surrounding organism. But evolution
can also generate surprises in the same domain. Prior to the evolution of eyes, who
would have predicted that the optical transparency of several enzymes and proteins
might one day become relevant to their suitability for cooptation as lens crystallins?)
Thus, even the simplest and universal geometries of filling space must generate
a host of spandrels to accompany any basically adaptive style of growth or
biomechanical form. Moreover, by using criteria of direct historical records (in
infrequent but best cases) or inference of genealogical order from cladistic
reconstructions based on living species (a strong, if indirect, mode of argument
almost always potentially available), we should be able to divide the useful features
of organisms into direct adaptations, coopted adaptations with different original uses,
and coopted spandrels—with the last category embodying a crucial challenge to strict
adaptationist thinking: currently useful features with nonadaptive origins. Consider a
simple example where the geometric nature of the spandrel can easily be defined, and
where we may infer nonadaptive origin from the evidence of cladograms. (Examples
of this kind could probably be multiplied indefinitely, and for any organism, were
biologists more inclined to grant the subject more attention and explicit study):
All snails that grow by coiling a tube around an axis must generate a cylindrical
space, called an umbilicus, along the axis. The umbilicus may be narrow and entirely
filled with calcite (then called a columella). But the space often remains open,
especially in land snails. A few species use the open umbilicus as a brooding chamber
to protect their eggs (Lindberg and Dobberten, 1981).
Is the umbilical brooding chamber a coopted spandrel—a space that arose as a
nonadaptive, geometric byproduct of winding a tube around an axis? Or